In many cases, Vertical Seismic Profiles (VSP) must be conducted with the source placed at considerable distance from the well. The reasons for this are usually ones of a practical nature; for example, source-induced noise such as casing ring and tube waves can obscure reflection events when the source is close to the well, the configuration of the drilling support equipment on a well pad may necessitate placement of a source at some distance from the well and culture or local land conditions may dictate the source placement. The most frequent demand for long offset VSP data acquisition is the requirement for tube wave suppression. Empirically, it is found that the amplitude of tube waves diminishes with increasing source offset from a well. In addition, the time delay of seismic reflection events resulting from a long offset source is usually less than the delay of tube wave inception. This tends to keep reflection events well separated from tube waves, resulting in much better VSP data quality.
Borehole seismic acquisition programs can be designed to obtain data at multiple long offsets for a number of reasons; the applications include: the provision of offset seismic profiles recorded in a well that are directly correlative to well log data, the acquisition of seismic data free of surface organized noise, the determination of seismic transmission and reflection properties with offset and depth, acquisition of data appropriate for tomography/inversion and imaging, the delineation of reservoir properties when offset VSP profiles are conducted in a field for several wells with 2-D or 3-D applications and determination of depth and inter-reflector velocities and reflectors below the borehole Total Depth (TD) from lookahead VSP data.
For purposes of this application, the terms, offset VSP, lateral seismic profile, and offset seismic profile, in a well as equivalent terms, meaning either seismic data acquisition for a multiplicity of receivers in a well recording data from each of a multiplicity of sources on the surface; or the reciprocal situation for which a multiplicity of receivers on the surface record data from each of a multiplicity of source locations in the well.
Offset VSP processing has not been as extensively developed as surface seismic data processing or conventional VSP processing. Conventional VSP data processing assumes that the source offset from the well is essentially zero. One of the greatest concepts of this century for reflection seismology has been Common Depth Point (CDP) stacking. This concept utilizes the near-hyperbolicity of primary seismic reflection arrivals to align redundant reflections obtained from multiple shot/receiver experiments conducted on the earth's surface. Some geophysical contractors approach the problem of aligning reflections from offset VSP data through ray tracing. Surface seismic data may be moveout corrected by ray tracing, but it is far more common to employ a hyperbolic moveout correction formula and develop velocity spectra in the applications.
The asymmetry of the shot and receiver locations in offset VSP precludes the possibility of CDP stacking exactly as in-surface seismic data processing.